It is usually desirable that a hearing aid provide good non-directional response to sounds emanating from points to the front, rear and sides of a user. A particularly effective nondirectional hearing aid is described in U.S. Pat. No. 3,201,528, assigned to the assignee of the present invention, wherein both forward facing and rearward facing sound receiving apertures are provided to direct received sound information to a microphone and to provide substantially uniform gain for sounds emanating from various points about the user. In many instances, however, such as in listening to lectures, conversations in a noisy environment, and the like, it would be desirable to enhance the directional characteristics of the hearing aid to accentuate the gain of sounds emanating forwardly of the user. Because of different needs in different listening situations, it is useful to provide a capability of both non-directional and directional characteristics in one hearing aid. A particularly effective selectably directional hearing aid is described in U.S. Pat. Nos. 3,909,556, assigned to the assignee of the present invention.
The invention described in the aforesaid patent attains two modes of operation by selectably occluding the rear aperture of the hearing aid by means of an acoustic valve, but at the same time keeping the rear transducer port energized to avoid undesirable instability. The valve is disposed in a network of sound passages connecting the front and rear sound receiving apertures to first and second input ports of a differential transducer. An acoustic filter in the second transducer port path is used to match the frequency and phase characteristics of the hearing aid response in the two modes so that switching from one mode to another does not materially alter the quality of sound reception. Hearing aids of this type have been found to be very useful. However, under certain operating conditions, especially in a high frequency sound environment, it has been found that the transducer can ring or otherwise become unstable.